The Dancing Mouse: A Study in Animal Behavior

by Robert Mearns Yerkes

CHAPTER XI

THE ROLE OF SIGHT IN THE DAILY LIFE OF THE DANCER

Darting hither and thither in its cage, whirling rapidly, now to the left, now to the right, running in circles, passing through holes in the nest box quickly and neatly, the dancer, it would seem, must have excellent sight. But careful observation of its behavior modifies this inference. For it appears that a pair of mice dancing together, or near one another, sometimes collide, and that it is only those holes with which the animal is familiar that are entered skillfully. In fact, the longer one observes the behavior of the dancer under natural conditions, the more he comes to believe in the importance of touch, and motor tendencies. Sight, which at first appears to be the chief guiding sense, comes to take a secondary place. In this chapter it is my purpose to show by means of simple experiments what part sight plays in the dancer's life of habit formation.

The evidence on this subject has been obtained from four sources: (1) observation of the behavior of dancers in their cages; (2) observation of their behavior when blinded; (3) observation of their behavior in a great variety of discrimination experiments, many of which have already been described; and (4) observation of their behavior in labyrinth experiments which were especially planned to exhibit the importance of the several kinds of vision which the dancer might be supposed to possess. The evidence from the first three of these sources may be presented summarily, for much of it has already appeared in earlier chapters. That from the fourth source will constitute the bulk of the material of this chapter.

My observation of the behavior of the mice has furnished conclusive evidence of their ability to see moving objects. But that they do not see very distinctly, and that they do not have accurate perception of the form of objects, are conclusions which are supported by observations that I have made under both natural and experimental conditions. In Chapters VII, VIII, IX, and X, I have presented an abundance of evidence of brightness vision and, in addition, indications of a specific sensitiveness to wave length which may be said to correspond to our color vision. It is noteworthy, however, that all of the experimental proofs of visual ability were obtained as the result of long periods of training. Seldom, indeed, in my experience with them, have the dancers under natural conditions exhibited forms of activity which were unquestionably guided by vision.

It is claimed by those who have experimented with blinded dancers that the loss of sight decreases the amount and rapidity of movement, and the ability of the animals to avoid obstacles.

By means of the discrimination method previously used in the preliminary experiments on color vision, a full description of which may be found in Chapter IX, p. 133, the dancers' ability to perceive form was tested. Immediately after the two males A and B had been given the "food-box" tests, whose results appear in Table 15, they were tested in the same apparatus and by the same method for their ability to discriminate a rectangular food-box from a round one. In the case of the color discrimination tests, it will be remembered that the circular tin boxes 5 cm. in diameter by 1.5 cm. in depth, one of which was covered with blue paper, the other with orange, were used. For the form discrimination tests I used instead one of the circular boxes of the dimensions given above and a rectangular box 8.5 cm. long, 5.5 cm. wide and 2.5 cm. deep. "Force" was placed in the circular box. The tests were given, in series of 20, daily.

TABLE 30

VISUAL FORM TESTS

SERIES DATE MOUSE A MOUSE B
RIGHT WRONG RIGHT WRONG
(CIRCULAR (RECTANGU- (CIRCULAR (RECTANGU-
BOX) LAR BOX) BOX) LAR BOX)
1 Jan. 5 10 10 9 11
2 7 12 8 13 7
3 10 6 14 10 10
4 11 7 13 10 10
5 12 9 11 10 10
6 13 11 9 11 9
7 14 13 7 9 11
8 15 10 10 11 9
9 16 10 10 11 9
10 17 11 9 9 11
11 18 11 9 12 8
12 19 12 8 10 10
13 20 10 10 12 8
14 21 10 10 8 12
15 22 10 10 10 10

Totals 152 148 155 145

The results of 15 series of these tests, as may be seen by the examination of Table 30, are about as definitely negative, so far as form discrimination is in question, as they possibly could be. From the first series to the last there is not one which justifies the inference that either of the dancers depended upon the form of the boxes in making its choice. In view of the general criticisms I have made concerning the use of hunger as a motive in experiments on animal behavior, and in view of the particular criticisms of this very method of testing the discriminating powers of the mouse, it may seem strange that space should be given to a report of these tests. I sympathize with the feeling, if any one has it, but, at the same time, I wish to call attention to the fact that almost any mammal which is capable of profiting by experience, and which, under the same conditions, could distinguish the rectangular box from the circular one, would have chosen the right box with increasing accuracy as the result of such experience. The results are important in my opinion, not because they either prove or disprove the ability of the dancer to discriminate these particular forms, the discrimination of which might fairly be expected of any animal with an image-forming eye, but because they demonstrate an important characteristic of the dancing mouse, namely, its indifference to the straightforward or direct way of doing things.

Most mammals which have been experimentally studied have proved their eagerness and ability to learn the shortest, quickest, and simplest route to food without the additional spur of punishment for wandering. With the dancer it is different. It is content to be moving; whether the movement carries it directly towards the food is of secondary importance. On its way to the food-box, no matter whether the box be slightly or strikingly different from its companion box, the dancer may go by way of the wrong box, may take a few turns, cut some figure-eights, or even spin like a top for seconds almost within vibrissa-reach of the food-box, and all this even though it be very hungry. Activity is pre-eminently important in the dancer's life.

In passing I may emphasize the importance of the fact that at no time did the brightness or color discrimination tests furnish evidence of attempts on the part of the dancers to choose by means of slight differences in the form of the cardboards or the cardboard carriers. Several times form differences, which were easily perceivable by the human subject, were introduced in order to discover whether the mice would detect them and learn to discriminate thereby instead of by the visual conditions of brightness or color. As these experiments failed to furnish evidences of form discrimination, the following special test in the discrimination box was devised.

[Illustration: FIGURE 22.—Cards used for tests of form discrimination.]

The color discrimination box of Chapter X was arranged so that the light at the entrance to each electric-box had a value of 20 candle meters, less the diminution caused by a piece of ground glass which was placed over the end of the electric-boxes to diffuse the light. The windows through which the light entered the electric-boxes were covered with pieces of black cardboard; in one of these cardboards I had cut a circular opening 4 cm. in diameter, and in the other an opening of the same area but markedly different shape. These openings are shown in Figure 22. As the mouse approached the entrance to the electric-boxes, it was confronted by these two equally illuminated areas, whose chief difference was one of form. Difference in the amount of light within the boxes was excluded so far as possible. The question which I asked was, can the dancer discriminate by means of this difference in visual form?

For the purpose of settling this point and of gaining additional knowledge of the role of vision, two individuals were tested in the discrimination box under the conditions which have just been described. During the first ten days of the experiment each of these mice, Nos. 420 and 425, was given a series of ten tests daily. At the end of this period experimentation with No. 425 had to be discontinued, and the number of daily tests given to No. 420 was increased to twenty.

Instead of taking space for the presentation of the daily records, I may state the general results of the tests. Neither of the mice learned to choose the right box by means of form discrimination. In fact, there was absolutely no sign of discrimination at any time during the tests. This result is as surprising as it is interesting. I could not at first believe that the mice were unable to perceive the difference in the lighted areas, but assumed that they were prevented from getting the outlines of the areas by the blinding effect of the light. However, decreasing the intensity of the illumination did not alter the result. According to the indications of this experiment, the dancer's ability to perceive visual form is extremely poor.

Thus far the purpose of our experiments has been to ascertain what the dancer is enabled to do by sight. Suppose we now approach the problem of the role of this sense by trying to find out what it can do without sight.

[Illustration: FIGURE 23.—Labyrinth B. I, entrance; O, exit; 1, 2, 3, doorways between alleys.]

For the investigation of this matter the labyrinth method seemed eminently suitable. The first form of labyrinth which was used in these visual tests appears in ground plan in Figure 23. It was made of 1-1/2 cm. boards. The length was 52 cm., the width 17 cm., the depth 10 cm. Each of the doorways, I (the entrance), 1, 2, 3, and O (the exit), was 5 by 5 cm. The alleys were 2-1/2 cm. wide. For this width the necessity is obvious from what has already been said of the animal's propensity to whirl on all occasions. As the mice almost never tried to climb up the walls, no cover for the labyrinth was needed. The direct route is indicated by the symbols I-1-2-3-O. If an error be defined as a choice of the wrong path as the animal progressed toward the exit, five mistakes were possible in the forward course: the first by turning to the left at the entrance; the second by failing to pass through doorway 1; the third by turning to the right after passing through doorway 1; the fourth by failing to pass through doorway 3, and the fifth by turning to the left after passing through 3. In case the mouse retraced its course, any mistakes made as it again progressed towards O were counted, as at first, no matter how many times it went over the same ground. Thus an individual might make the same mistake several times in the course of a single test in the labyrinth.

With this labyrinth Nos. 7, 998, 15, 16, 151, and 152 were tested. At first a record was kept of the time which elapsed from the instant the animal entered I to the instant it emerged at O, of the path which it followed, and of the number of errors which it made; but later only the number of errors was recorded.

TABLE 31

THE ROLE OF SIGHT

Labyrinth-B Experiments

NO. 7 NO. 998

TEST DATE TIME ERRORS TIME ERRORS
1 June 16 66" 8 127" 19
2 16 11 0 94 12
3 16 15 2 18 3
4 16 7 0 13 2
5 16 5 0 10 1
6 18 61 15 12 3
7 18 13 3 14 4
8 18 14 5 8 1
9 18 24 9 16 2
10 18 10 1 9 1
11 19 36 13 80 17
12 19 8 3 10 1
13 19 6 1 7 1
14 19 9 1 8 0
15 19 12 2 7 0
16 20 14 1 25 0
17 20 28 3
18 20 No efforts No efforts
to escape to escape